Place butter in a 13 x 9 casserole dish and melt in a 350 degree oven (about 5-8 minutes).
While the butter melts, combine 3/4 c. flour, 1 c. sugar, 1/2 t. salt and 2 t. baking powder. Pour in the milk and vanilla and stir just until combine
In a second bowl, toss the peach cubes, blueberries, 1/4 c. sugar, 1 T. flour and 1/2 t. cinnamon together.
Once butter is melted, remove casserole dish from oven. Pour batter into casserole dish. Pour fruit mixture on top. Return to 350 degree oven and bake for about 45 minutes, until golden brown and crisp on top.